Sync with address book

Ok guys this is gunna sound kind of weird, but I've always been afraid to click the sync with address book box in iTunes for some reason. Back in June before the iPhone release I used iSync to get my contacts from my blackberry pearl to my MBP. Then they were automatically placed on my iPhone (I think). But now I always have that unclicked box starring at me when I'm in iTunes! So I guess my question is, is there any point in clicking it? I'm I missing out on anything? And, most importantly, will I lose any info or any of my backups if I do? Thanks in advance